ORIGINAL: Bax
Usually, the case had a small ''SR'' just under the exhaust port. Because the center crankcase was identical in the FSR and RSR models, it would not have an ''F'' or ''R'' cast into the case.
Just a minor point there Bax but the original crankcase casting was made for the first Schneurle ported OS which was the pylon/rat racing rear drum valved OS 40SR which probably indicated Schneurle port Rear induction (maybe even Racing). The same crankcase was then used for the Front induction sports engine so the engines were then called FSR although still keeping the SR under the exhaust port. I don't know if the 45FSR kept the SR or whether it was replaced by a 45 under the exhaust.